Doug is conducting a port scan of a target network.

If the destination host or the destination network is down there is no way to get an answer and if TTL (Time To Live) is set too low the UDP packets will “die” before reaching the host because of too many hops between the scanning computer and the target. The TCP receive window size is the amount of received data (in bytes) that can be buffered during a connection. The sending host can send only that amount of data before it must wait for an acknowledgment and window update from the receiving host and ICMP is mainly used for echo requests and not in port scans.